Orrorin

Orrorin tugenensis Temporal range: Miocene
Orrorin tugenensis fossils
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Subfamily:	Homininae
Tribe:	Hominini
Subtribe:	Hominina
Genus:	Orrorin Senut et al. 2001
Species:	O. tugenensis
Binomial name
†Orrorin tugenensis Senut et al. 2001

Orrorin tugenensis is considered to be the second-oldest (after Sahelanthropus) known hominin ancestor that is possibly related to modern humans, and it is the only species classified in genus Orrorin. Orrorin is significant because it can be an early bipedal hominin.

The name ^[1] was given by the discoverers who found Orrorin fossils in the Tugen Hills of Kenya in 2000. ^[2]

By analysing radiometric decay (K–Ar dating), paleomagnetism, and biochronology the age of the specimen have been estimated to 6 to 5.8 million years (Ma) (average: 6.1 Ma). At present, 20 fossils have been found at four sites in the Lukeino Formation: of these, the fossils at Cheboit and Aragai are the oldest (6.1 Ma), while those in Kapsomin and Kapcheberek are found in the upper levels of the formation (5.7 Ma). ^[3]

Fossils

The 20 specimen found this far include: the posterior part of mandible in two pieces; a symphysis and several isolated teeth; three fragments of femurs; a partial humerus; a proximal phalanx; and a distal thumb phalanx. ^[3]

Orrorin had small teeth relative its body size. Its dentition differs from that found in Australopithecus' in that its cheek teeth are smaller and less elongated mesiodistally; and from Ardipithecus in that its enamel is thicker. The dentition differs from both these species in the presence of mesial groove on the upper canines. The canines are ape-like but reduced, like those found in Miocene apes and female chimpanzees. Orrorin had small post-canines and was microdont like modern humans, whereas Australopithecus was megadont. ^[3]

In the femur, the head is spherical and rotated anteriorly; the neck is elongated and oval in section; and the lesser trochanter protrudes medially. While this suggest that Orrorin was bipedal, the rest of the postcranium indicates it climbed trees. while the proximal phalanx is curved, the distal pollical phalanx is of human proportions and have thus been associated with toolmaking, but should probably be associated with grasping abilities useful for tree-climbing in this context. ^[3]

After the fossils were found in 2000, they were held at the Kipsaraman village community museum, but the museum was subsequently closed. Since then, according to the Community Museums of Kenya chairman Eustace Kitonga, the fossils are stored at a secret bank vault in Nairobi.^[4]

Classification

If Orrorin proves to be a direct human ancestor, then australopithecines such as Australopithecus afarensis ("Lucy") may be considered a side branch of the hominid family tree: Orrorin is both earlier, by almost 3 million years, and more similar to modern humans than is A. afarensis. The main similarity is that the Orrorin femur is morphologically closer to that of H. sapiens than is Lucy's; there is, however, some debate over this point. ^[5]

Other fossils (leaves and many mammals) found in the Lukeino Formation show that Orrorin lived in dry evergreen forest environment, not the savanna assumed by many theories of human evolution.^[5]

Discovery

The team that found these fossils in 2000 was led by Brigitte Senut and Martin Pickford^[6] from the Muséum national d'histoire naturelle. The discoverers conclude that Orrorin is a hominin on the basis of its bipedal locomotion and dental anatomy; based on this, they date the split between hominins and African great apes to at least 7 million years ago, in the Messinian. This date is markedly different from those derived using the molecular clock approach, but has found general acceptance among paleoanthropologists.

See also

• Praeanthropus
• Martin Pickford

Notes

1. ^ Tugenensis from "Tugen Hills", and Orrorin from local language "original man"; it was also nicknamed the "Millenium Man".(Haviland et al. 2007, p. 122)
2. ^ Haviland et al. 2007, p. 122
3. ^ ^{a b c d} Henke 2007, pp. 1527–9
4. ^ "Whereabouts of fossil treasure sparks row". Daily Nation. May 19 2009. http://www.nation.co.ke/News/-/1056/600372/-/ujn367/-/index.html. Retrieved December 2010. 
5. ^ ^{a b} Pickford 2001, Interview
6. ^ Senut et al. 2001

References

• CogWeb. "Orrorin Tugenensis: Pushing back the hominin line". UCLA. http://cogweb.ucla.edu/ep/Orrorin.html. Retrieved December 2010. 
• Haviland, William A.; Prins, Harald E. L.; Walrath, Dana; McBride, Bunny (2007). Evolution and prehistory: the human challenge. Cengage Learning. ISBN 049538190X. http://books.google.com/books?id=LfYirloa_rUC&pg=PA122&dq=Orrorin. 
• Henke, Winfried (2007). Henke, Winfried; Hardt, Thorolf; Tattersall, Ian. eds. Handbook of paleoanthropology: Phylogeny of hominids. Springer. pp. 1527–9. ISBN 9783540324744. http://books.google.com/books?id=vhoRdbTrjc8C&pg=PA1527&dq=Orrorin. 
• Pickford, Martin (December 2001). "Martin Pickford answers a few questions about this month's fast breaking paper in field of Geosciences". Essential Science Indicators. http://www.esi-topics.com/fbp/comments/december-01-Martin-Pickford.html. 
• Senut, Brigitte; Pickford, Martin; Gommery, Dominique; Mein, Pierre; Cheboi, Kiptalam; Coppens, Yves (2001). "First hominid from the Miocene (Lukeino Formation, Kenya)". Comptes Rendus de l'Académie de Sciences 332 (2): 137–144. Bibcode 2001CRASE.332..137S. doi:10.1016/S1251-8050(01)01529-4. http://www2.ku.edu/~lba/Anth703/Articles/ORRORIN.pdf. Retrieved December 2010. 

External links

• Orrorin tugenensis - The Smithsonian Institution's Human Origins Program